This application is a divisional of U.S. patent application Ser. No. 10/389,465, filed on Mar. 13, 2003 now U.S. Pat. No. 6,818,361, which is a continuation of U.S. application Ser. No. 09/430,689, filed on Oct. 29, 1999, now U.S. Pat. No. 6,562,522; the disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application.
This disclosure relates to photomasking.
Optical lithography is a technology used to print patterns that define integrated circuits onto semiconductor wafers. Typically, a pattern on an attenuation photomask is imaged by a highly accurate camera. The image is transferred onto a silicon wafer coated with photoresist. Continued improvements in optical lithography have enabled the printing of ever-finer features. This has allowed the integrated circuit industry to produce more powerful and cost-effective semiconductor devices.
A conventional binary mask that controls the amplitude of light incident upon a wafer is often inadequate when the integrated circuit (IC) feature size is small.
Phase-shifting masks can be used for optical lithography for the generation of IC feature sizes below one micron such as 0.25 micron. Under these subwavelength conditions, optical distortions as well as diffusion and loading effects of photosensitive resist and etch processes cause printed line edges to vary. Phase shifting improves the resolution that optical lithography can attain, producing smaller, higher-performance IC features by modulating the projected light at the mask level.
Successors to optical lithography are being developed to further improve the resolution. Extreme-ultraviolet (EUV) lithography is one of the leading successors to optical lithography. It may be viewed as a natural extension, since it uses short wavelength optical radiation to carry out projection imaging. However, EUV lithography (EUVL) technology is different from the technology of optical lithography in that the properties of materials with respect to EUV radiation are different from their properties with respect to visible and UV ranges. For example, the EUV radiation is strongly absorbed in virtually all materials, including gas. Thus, EUVL imaging systems often utilize entirely reflective optical elements rather than refractive elements, such as lenses.